1. Field of the Invention
The present invention relates to a method of preparing granules of a dipeptide. More particularly, the invention relates to a method of preparing granules of a dipeptide sweetener having high bulk density which may be handled with ease.
.alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter referred to as .alpha.-APM) is one preferred component of the sweetener granules prepared by the present invention. .alpha.-APM is a low-calorie sweetener having a sweetness about 200 times that of sucrose, and the worldwide demand for this sweetener is anticipated to be over 10,000 tons per year before 1995.
2. Description of the Related Art
Where .alpha.-APM is used in powder form, various problems arise due to the intrinsic characteristics of the powder. For example, .alpha.-APM powder easily scatters and easily adheres to the walls of vessels, devices, etc. making its handling difficult. In addition, .alpha.-APM powder has low dispersibility and low solubility in water, making dissolution difficult and accompanied by foaming.
One known means for improving the characteristics of .alpha.-APM is a method of granulating .alpha.-APM. Both wet extrusion granulation (Japanese Patent Application Laid-Open No. 59-95862) and dry compaction granulation (Japanese Patent Publication No. 1-15268) have been described. From the point of view of reducing thermal load and providing a simpler process, dry compaction granulation is preferred.
In dry compaction granulation of .alpha.-APM, .alpha.-APM is first compacted into a sheet, briquette or the like, which is then broken and milled into granular .alpha.-APM. During the breaking and milling, however, a part of the .alpha.-APM is powdered. Thus, the product actually obtained by dry compaction is a mixture of a granular .alpha.-APM and fine, powdery .alpha.-APM.
In order to overcome the above-mentioned problems, the fine powder formed during breaking and milling of compacted .alpha.-APM must be removed. In addition, especially from the viewpoint of improving the solubility of the .alpha.-APM granules formed, the larger granules must also be removed. Specifically, it is desired that .alpha.-APM be granulated to have a particle size falling within the range of from 100 .mu.m to 1400 .mu.m.
In compaction granulation generally, a granulation process is employed for the purpose of obtaining a final granular product having a particle size falling within a desired particle size range in which (1) after a raw powder has been compacted or after the compacted material has been roughly broken, the compacted material or the roughly broken material is broken down or further broken down until the material can pass through a screen or perforated plate having pores with slightly greater than or approximately the same diameter as the uppermost limit of the intended or desired particle size range of the granules, and (2) thereafter the material having passed through the screen or plate is sieved through a sieve having pores with approximately the same diameter as the lowermost limit of the intended or desired particle size range of the granules, and the granules that remain on the sieve are collected as the final product. During sieving step (2), the material which passes through the sieve is recycled to the initial compacting step and is reused.
However, when .alpha.-APM is granulated according to this process a problem arises in that the percentage .alpha.-APM that is overmilled into fine powder in step (1) above where the compacted .alpha.-APM is broken and milled so as to pass through the pores of a screen or plate having approximately the desired maximum particle size is extremely high. This means that the ratio of the amount of the final granular product to the total amount of the compacted .alpha.-APM (hereinafter referred to as a "one-pass yield") is low while the ratio of the amount of the fine powdery .alpha.-APM which must be recycled is high. Where the one-pass yield is low, the amount of material which must be processed in order to obtain a predetermined amount of the final granular product increases dramatically, causing inefficient reprocessing and the necessity of equipment which is capable of handling large amounts of material.
The addition of a binder such as water, alcohol or the like to .alpha.-APM during compaction would be effective for preventing formation of fine powdery .alpha.-APM to some degree. However, the presence of, or any increase in the amount of, binder added results in the necessity of a step of drying the .alpha.-APM granules obtained, which is unfavorable from an efficiency and/or industrial viewpoint.
In general, a shaking or vibrating sieving machine is employed in step (2) above for removing fine powder by sieving. However, since .alpha.-APM powder is very easy to aggregate, it would aggregate on such a shaking or vibrating sieve and would not pass through the sieve. Thus, the use of such a shaking or vibrating sieving machine for the treatment of .alpha.-APM involves an additional problem. Lowering of the sieving efficiency results in the problem that the final granular product contains fine powder. In order to avoid this problem, the necessary sieving area must be enlarged, which results in the necessity of enlarging the necessary equipment.